Decades after a teenager’s brutal murder in northwestern Washington left detectives lost for answers, modern forensic tests have helped identify her killer.

Tracy Whitney’s body was found by fisherman in the Puyallup River, near the city of Sumner, on Aug. 28, 1988. She was nude, and the subsequent autopsy that confirmed Whitney’s name also determined her cause of death to be asphyxia from strangulation and probable smothering. She was believed to have been sexually assaulted, which additional autopsy findings supported, according to the Pierce County Sheriff’s Department, which operates in the area around Tacoma and investigated Whitney’s case.

Whitney was 18 when she died. At the time, Pierce County detectives tried to develop leads as they searched for whomever was responsible in her murder. They interviewed people who knew Whitney, and people who had dated her, as the probe got underway. But despite collecting DNA samples from the body that presumably belonged to the killer, investigators were unable to pinpoint a suspect.

The case remained unsolved for years. In 2005, another effort to find the suspect by running those DNA samples through CODIS — a national database that law enforcement can use to link forensic profiles to crimes and suspects called the Combined DNA Index System — ultimately proved unsuccessful as no matches were found, according to the Pierce County sheriff.

It wasn’t until 2022, owing to a grant from the Washington State Attorney General’s Office, that a Pierce County sheriff’s cold case detective submitted the DNA to a genetic genealogy laboratory for repeat testing. Genetic Genealogy is a process where forensic analysts build a family tree from a DNA profile, which in this case allowed law enforcement to identify Whitney’s killer by way of a family member.

Findings from the genealogy lab led detectives to the biological son of their suspect in Whitney’s death, John Guillot, Jr., and confirmed that the elder Guillot was in fact a match for the DNA, according to the Pierce County sheriff. Guillot, the suspect, died from cancer in January 2022, just eight months before the lab returned the DNA match that implicated him in the murder.

“There were no connections between Tracy and Guillot Jr and detectives believe this was a stranger abduction, rape and murder,” the Pierce County Sheriff’s Office said in a social media post Saturday announcing Whitney’s case had been solved. “Our Cold Case Unit is always looking into the past trying to find answers for families just like Tracy’s. We are glad they can finally have an answer and some closure with the solving of this case.”

Although Whitney’s killer could not be prosecuted, her relatives told CBS News affiliate KIRO that knowing he has been identified does bring them some sense of peace. Her sister, Robin Whitney, said she hopes identifying Guillot publicly helps solve other cold cases, as she believes he may have committed multiple crimes during his life.

“We believe that John Guillot Jr. likely committed other crimes,” said Robin Whitney in a social media post, according to KIRO. “However, since he is deceased and due to current legal restrictions, his DNA cannot be uploaded into CODIS (law enforcement DNA database). We hope that this policy can be reevaluated, as it could aid in solving additional cases.”